In the very beginning, movies were only broadcasted by TV stations with audio signals in the original language or were provided with an entirely translated version of the audio signal. Later on subtitles were added as translation for foreign language movies but also parallel to the audio signal in the same language for the benefit of the hard of hearing viewing community. The possibility to listen to a foreign language while being able to read its translation into the mother tongue has made subtitling also a tool for language learning.
Subtitles can be provided as so-called open subtitles, which are put into the TV picture already at the TV station before the TV signal is transmitted. This enables the viewers to see subtitles without the need of a special decoder in the TV set. However, as a result only one language can be transmitted at a time and the subtitles cannot be switched off. Nowadays more popular is the use of so-called closed captioning, where the subtitles are transmitted in a hidden part of the TV picture. For analog TV signals the closed captioning subtitles are inserted in the Vertical Blanking Interval (VBI) between the frames of the TV signal and are transmitted e.g. in Europe as Teletext data, in the US as line 21 data. At the receiving end the subtitling data are extracted from the VBI by a data slicer, decoded using a Teletext or Closed Captioning Decoder and displayed together with the video content if called up by the viewer. This way it is possible for each viewer to decide whether he wants to see subtitles or not.
Similarly, digital TV signals may comprise subtitles. Especially digital satellite TV allows satellite broadcasters to transmit the program with more than one language for audio and subtitles, giving the viewer the possibility to choose the audio language and the subtitles in a language he wants to have.
Movies and other broadcasts including subtitles may be recorded by a viewer using a video cassette recorder or other recording devices. This allows replaying the whole movie or parts of it, which is especially helpful for language learning purposes. Furthermore, not only broadcasted movies may come with subtitle information but also pre-recorded movies. Especially, movies pre-recorded on DVD (Digital Versatile Disc) usually offer at least two spoken languages and many subtitle languages. This allows the DVDs to be widely distributed without producing different language specific versions for each individual country and makes them also a very efficient media for learning a foreign language.